The present invention relates generally to cryogenic preservation, and more particularly to heat exchange media used in cryopreservation.
Cryopreservation refers to all stages of preservation: treatment, freezing, storage, and thawing processes. Considerable research efforts have been devoted to developing cryoprotective substances, as well as to optimization of freezing and thawing temperatures and cooling rates for various cell types and materials. Other sectors of this research effort have focused on heat transfer compounds and heat transfer mechanisms within the temperature domain of cryogenic preservation.
Heat transfer processes move thermal energy to or from an object in physical contact with a heat transfer fluid which is either at a temperature hotter or colder than the object. Various organic fluids have been used as such heat transfer fluids for high temperature (non-cryogenic) heat transfer processes. In the low temperature domain of cryogenics, low molecular weight alcohols, ketones and halogenated hydrocarbons have been used for low temperature heat transfer processes.
Low temperature heat transfer processes continue to have difficulties caused by the volatility, toxicity, flammability, foaming, or low temperature viscosity changes of conventional low temperature organic heat transfer fluids. Some conventional low temperature heat transfer fluids, such as acetone, absorb any moisture they contact. A heat transfer apparatus employing such fluids may thus adversely affect low temperature heat transfer processes. The efficiency of the thermal energy transfer process is also adversely impacted by viscosity increases and gelation of the low temperature heat transfer fluid, as reduced circulation or clogging of parts of the heat transfer apparatus can occur. Additionally, the rate at which these heat transfer fluids absorb heat energy is generally less than optimal.
Therefore, what is needed is an improvement in heat transfer processes in the cryogenic realm which avoid the problems previously discussed. Accordingly, the various embodiments of the present invention disclose methods for producing a pre-conditioned solute with more efficient heat transfer properties, in addition to other utile capabilities and characteristics in a cryogenic process. For example, the solutes disclosed herein do not exhibit an increase in temperature during a latent heat phase transition when used in a freezing process, or, at the very least, exhibit a reduced increase in temperature.
In an embodiment, a solute is pre-conditioned by being super-cooled from ambient room temperature to about xe2x88x9223 degrees C. very quickly, on the order of at least about 6.5 degrees C. per minute, on average. This rapid chilling of the solute results in a super-cooled solute, which may then be used as a heat exchange medium to absorb heat from substances immersed in the pre-conditioned solute. Super-cooling is cooling a liquid substance below the freezing point without solidification or crystallization taking place. Super-cooling alters a heat absorption rate of the solute such that pre-conditioned solute has an increased heat absorption rate in comparison to solute which has not been pre-conditioned. The heat absorption rate of a pre-conditioned solute according to one embodiment of the present invention is about 135 BTU at a temperature of between about xe2x88x9223 degrees C. and xe2x88x9226 degrees C.
In an embodiment, pre-conditioning a solute includes super-cooling the solute from ambient room temperature to between about xe2x88x9223 degrees C. and xe2x88x9226 degrees C. at an average rate of cooling of between about 6.5 degrees C. and 8.5 degrees C. In a further embodiment, the step of pre-conditioning the solute includes super-cooling the solute, for at least a portion of time, at an average cooling rate of at least about 17 degrees C. per minute.
After super-cooling, a portion of the pre-conditioned solute remains in a super-cooled state after being pre-conditioned as disclosed herein. In this super-cooled state, the heat that normally would be released upon freezing of the solute is decreased, thus the pre-conditioned solute exhibits no spike in temperature upon subsequent cooling from ambient room temperature to between about xe2x88x9223 degrees C. and xe2x88x9226 degrees C. The pre-conditioned solute can be used as the cooling liquid in a system consisting of a tank capable of holding a predetermined amount of liquid, a circulator to circulate the liquid in the tank, and a refrigeration system capable of cooling the liquid within the tank.
An object of at least one embodiment of the present invention is to produce a solute with improved heat absorption properties for use in a cryogenic process.
An advantage of at least one embodiment of the present invention is that the heat absorption rate of pre-conditioned solute is greater than the heat absorption rate as compared to a non-conditioned solute, making the pre-conditioned solute a better heat exchange medium than a non-conditioned solute.
A further advantage of at least one embodiment of the present invention is that freeze damage to sensitive materials is decreased because no temperature spike is observed in a pre-conditioned solute upon subsequent freezing.